The advancement of technology in the field of digital signal recording has resulted in a continuing increase in areal recording density. In the field of magnetic recorders, a transition is not immediately made by all users from the old to the new as improved equipment capable of accommodating higher densities is made available. Users of older equipment may often choose to defer new acquisitions and use their existing equipment for the duration of its useful life. The transition from the old to new is therefore gradual and at any given instant of time, there is a spectrum of digital tape recorders of varying ages in the marketplace. The situation is dynamic in that new machines of increased capability are periodically introduced so that a new machine representing the present state of the art is destined to be replaced ultimately by improved machines at some future time.
It is desirable that newer machines have the capability of backward compatibility. Backward compatibility refers to the ability of a newer machine capable of reading and writing with higher signal densities to be able to read and/or write tapes produced on earlier machines of lower recording densities. It is often the case that tapes or the like (hereinafter tapes) recorded by a first machine are subsequently read by another machine. If the first machine has only a low density capability and the second machine has higher density capabilities, or vice versa, it is necessary that the tape be compatible to both machines so that the second machine can read the tapes generated by the first machine.
It is also desirable that the newer machines be flexible in their operation and have the capability of recording at different bit densities. This provides maximum compatibility and permits the use and exchange of tapes among the various recorders owned by a single user or family of users who desire to exchange tapes.
It is necessary that tape recorders capable of operating at a plurality of different flux densities know the density of the user information recorded on each tape that is to be read. It is known to record density information on a magnetic tape specifying the density at which user information is recorded on the tape. U.S. Pat. No. 4,054,947 discloses a computer-tape transport interface which can both record information on a tape indicating the recording density used to record user data, as well as reading density information from a priorly recorded tape. In accordance with the teachings of this U.S. patent, density information is recorded on the beginning portion of the tape in the form of a series of binary ones and zeroes. A burst of binary ones and zeroes specifies a first recording density. The absence of such a burst represents a different density.
U.S. Pat. No. 3,932,894 discloses a rotating head magnetic recorder adapted to be used with an open reel magnetic tape having a designated area reserved on the lead end of the tape for recording density information. This Patent does not teach the specifics of how density information is to be recorded. It merely teaches that an appropriate density code can be recorded on a tape specifying the bit density of the user data recorded elsewhere on the tape. Since the patent is directed to rotating head magnetic recording apparatus in which information is recorded in a plurality of angularly positioned tracks intermediate the sides of the tape, the density information is presumably recorded in similar angularly oriented tracks.
U.S. Pat. No. 4,298,897 discloses an improvement of U.S. Pat. No. 3,932,894 and, in particular, discloses a rotating head magnetic recorder having the capability of reading and writing tapes with different flux densities. This Patent discloses apparatus having a wide recording head and two separate relatively narrow reading heads. Data is recorded at lower densities by using the entire width of a portion of the tape termed a "stripe." The recording head is moved one stripe at a time, from stripe to stripe, to record successive stripes of low density information.
Only a half of a stripe is used for the recording of high density information. In this case, the recording head, is only moved a distance of one-half stripe at a time as successive half stripes are recorded. The apparatus of this patent always uses a recording head that is one stripe wide. The first stripe is recorded by using the entire width of the stripe. Next, the recording head is moved a distance of one-half a stripe and the recording process continues. This recording process for the second stripe overwrites one-half of the first stripe that was recorded. The process continues in this manner with high density information being recorded for the full width of a stripe but with one-half of a stripe being overwritten as the next stripe is recorded.
On playback, high density information is read using two read heads concurrently with the entire width of the stripe being read at a time and with each of the two read heads reading its assigned one-half of a stripe. Only a single read head is used for reading one stripe wide low density information. However, the second read head is available as a backup in case of a failure of the first read head. The recording apparatus of this U.S. Pat. No. 4,298,897 records density information in one end of each stripe. The density information (62 and 63) in a stripe comprises a two bit code. This patent also records a table of contents at the beginning of the tape (CTOC). The table of contents is recorded at low density and specifies the density used on the rest of the tape.
While the teachings of the above-identified U.S. Patents represent an advance in the art in that they provide arrangements for recording density information on a magnetic tape or the like, they each suffer from one or more disadvantages. They each use a single track to record the tape density information. U.S. Pat. No. 4,054,947 uses a single burst of information in a single track to indicate one possible tape density and uses the absence of this information to represent another density. This arrangement is disadvantageous because of inherent problems of reliability. First of all, the absence of density information could be due to a failure of the recording mechanism of the device that originally recorded the tape. The absence of a signal could also be caused by a blank or an alien tape. The presence of the ID burst is a positive indication, absence is not. Secondly, the presence of the information need not necessarily represent valid density information. Instead, it could possibly represent user data on a tape that does not contain any density information whatsoever. Also, only one binary condition is encoded by this patent--the presence of high density information.
U.S. Pat. Nos. 3,932,894 and 4,298,897 record the density information in the form of sequential binary digits on a single track or stripe of the tape. This arrangement is subject to problems of reliability in that false information due to noise and the like could mimic valid tape density information. Also, the tape density codes could be mimicked by user data on tapes that do not contain any density information whatsoever. An example of such tapes could be those recorded by an alien tape drive that does not follow these teachings. Also, schemes which require reading and writing of specific bit patterns are more difficult to implement. For example, if the ID is recorded at one density, a later machine designed for higher density would need to be able to encode, write, read, and decode ID data at this lower density. The facilities of U.S. Pat. No. 4,298,897 are also less than ideal in that they do not permit a single track of data to be overwritten in high density mode while leaving the remaining tracks unaltered. The reason for this is that the wide recording head of this Patent spans two half stripes and therefore the recording of a single high density track in a half stripe of the tape would overwrite valid data in the adjacent one-half stripe tape segment. It may therefore be seen that the currently available facilities for writing tape density information on a magnetic record such as a tape are less than ideal.